New South African Astronomical Observatory (SAAO) BV(RI)(C) measurements of 19 Hyades stars and 11 M67 stars are reported. The zero points of the new color indices conform closely to those of SAAO data reported in a previous paper. In addition, the new M67 measurements of (V - R)(C) and (R - I)(C) are compared to data published previously by Taylor, Joner, and Jeffery. The results support conclusions drawn by those authors that the scale factors of their data are correct and that a scale factor problem exists in measurements published by Montgomery, Marshall, and Janes. The new values of B - V are used with Tycho data in tests of extant Hyades and M67 measurements and of the accuracy of the SAAO B - V system. A problem encountered previously with the Hyades B - V zero point is resolved, and an extant Hyades relation between B - V and (R - I)(C) is re-zeroed. A satisfactory zero point is also obtained for M67, and published photomultiplier values of B - V are reduced to that zero point, averaged, and tabulated. It is found that the zero point of B - V data published by Sandquist is satisfactory. However, tests of B - V measurements made by Montgomery et al. suggest that those data are not on a single zero point. Finally, the scale factor of the E region B - V system is found to be satisfactory, but a well-supported interim conclusion is drawn that E region values of B - V should be corrected by about - 9 mmag. It is suggested that this conclusion be tested by using instrumental systems that have not yet contributed to the testing process.

Aims. We assess the evolutionary status of the intriguing object Walker 90/V590 Mon, which is located about 20 arcmin northwest of the Cone Nebula near the center of the open cluster NGC 2264. This object, according to its most recent optical spectral type determination (B7), which we confirmed, is at least 3 mag too faint in V for the cluster distance, but it shows the classical signs of a young pre-main sequence object, such as highly variable H alpha emission, Mg II emission, IR excess, UV continuum, and optical variability. Methods. We analyzed a collection of archival and original data on Walker 90, covering 45 years including photometry, imaging, and spectroscopic data ranging from ultraviolet to near-infrared wavelengths. Results. According to star formation processes, it is expected that, as this object clears its primordial surroundings, it should become optically brighter, show a weakening of its IR excess and present decreasing line emissions. This behavior is supported by our observations and analysis, but timescales are expected to be longer than the one observed here. Based on photometric data secured in 2007, we find Walker 90 at its brightest recorded optical magnitude ((12.47) over bar +/- 0.06). We document an evolution in spectral type over the past five decades (from A2/A3 to currently B7 and as early as B4), along with a decrease in the near-infrared K fluxes. From near-infrared VISIR images secured in 2004, Walker 90 appears as a point source placing an upper limit of < 0.1 '' for its diameter. Evidence of turbulent inflows is found in rapidly changing inverse P-Cygni profiles in the lower Balmer lines, with a broadening of +/- 400 km s(-1) in Ha and a redshifted component in H beta with a terminal velocity of similar to 600 km s(-1). The measured steep UV continuum fluxes (mimicking a star as early as B4), added to a tentative identification of N V emission, suggest a strong non-photospheric component, typically of fluxes arising from a thermally inhomogeneous accretion disk. We detect a well defined 2200 angstrom bump, indicative of dense material in the line-of-sight. We conclude that many observational features are explained if W90 is a flared disk system, surrounded by an inclined optically thick accretion disk.

In this paper, new Cousins VRI data are presented for NGC 752 and Praesepe, and new and extant data are combined into an augmented database for M67. For those three clusters, catalogs containing Cousins VRI photometry, reddening-corrected values of (V -K)(J), and temperatures are produced. The same is done for Coma by using both previously published and newly derived Cousins photometry. An extant set of catalogs for the Hyades is updated to include V magnitudes and values of (R -I)(C) that were published after the original catalogs appeared. Finally, M67 V magnitudes published previously by Sandquist are corrected for an effect that depends on location on the face of the cluster. The corrected data and values of (V -I)(C) given by Sandquist are then set out in a supplementary catalog. Data files containing all of these catalogs are deposited in the CDS archives. To assess the quality of the data in the catalogs, the consistency of extant Cousins VRI databases is tested by performing analyses with the following features: (1) quantities as small as a few millimags are regarded as meaningful; (2) statistical analysis is applied; (3) no use is made of data other than VRI measurements and comparable results; (4) no inferences are drawn from color-magnitude comparisons; (5) pertinent data that have not been included previously are analyzed; and (6) results based on direct comparisons of stellar groups at the telescope are featured. In this way, it is found that our updated M67 color data and those of Sandquist are on the E region zero point. In contrast, values of (V -I)(C) from Montgomery and collaborators are found to be too red by 27 +/- 3 mmag, with an even larger offset being likely for unpublished data from Richer and his collaborators. Zero-point tests of our Cousins VRI colors for Coma, Praesepe, and NGC 752 are also satisfactory. Scale factor tests of the M67 colors are performed, and a likely scale factor error in the Montgomery et al. colors is found. However, it appears at present that the scale factors of our M67 colors and those of Sandquist are satisfactory. For the most part, zero-point tests of the assembled V magnitudes are also satisfactory, although it is found that further work on the V magnitudes for Praesepe and M67 would be useful. To put these results in perspective, it is pointed out that photometric tests that are satisfactory at the few-millimag level have been published for some two decades and so are not appearing for the first time in this paper.

New Stromgren-beta Pleiades data on a zero point defined by the Hyades and Coma are presented. The new data and counterparts from four extant sources are compared to measurements published by Crawford & Perry in 1976. The only statistically significant problem found in this way is a 4 mmag offset obtained b - y from a data set published by McNamara in 1976. Results of similar comparisons published previously for Praesepe are updated by including three additional sets of published data. In this case, comparisons are made to measurements published by Crawford & Barnes in 1969. Two of the data sets (including one published by Joner & Taylor in 1995) yield accordant, statistically significant formal corrections to the b - y m(1) measurements of Crawford & Barnes. In addition, results from a third data set are consistent with those derived from the data of Joner & Taylor. However, this pattern is either partly or entirely absent from the offsets yielded by the other three data sets. Because the measurements of Joner & Taylor are still the only ones known to have been derived from direct comparisons among the Hyades, Coma, and Praesepe, it is concluded that the balance of evidence presently favors the offsets obtained from those data. In addition, it is noted that the data of Joner & Taylor do not imply that the data of Crawford & Barnes require corrections that depend on color. With these interim conclusions drawn, it is then suggested that further Praesepe measurements should be brought to bear on the problem.

In this paper, transformations between Cousins R - I and other indices are considered. New transformations to Cousins V - R and Johnson V - K are derived, a published transformation involving T-1-T-2 on the Washington system is rederived, and the basis for a transformation involving b-y is considered. In addition, a statistically rigorous procedure for deriving such transformations is presented and discussed in detail. Highlights of the discussion include ( 1) the need for statistical analysis when least-squares relations are determined and interpreted, ( 2) the permitted forms and best forms for such relations, ( 3) the essential role played by accidental errors, ( 4) the decision process for selecting terms to appear in the relations, ( 5) the use of plots of residuals, ( 6) detection of influential data, ( 7) a protocol for assessing systematic effects from absorption features and other sources, ( 8) the reasons for avoiding extrapolation of the relations, ( 9) a protocol for ensuring uniformity in data used to determine the relations, and ( 10) the derivation and testing of the accidental errors of those data. To put the last of these subjects in perspective, it is shown that rms errors for VRI photometry have been as small as 6 mmag for more than three decades and that standard errors for quantities derived from such photometry can be as small as 1 mmag or less.

New BV(RI)(C) observations of 77 stars in the Hyades are reported and discussed. The new observations are used to test published magnitudes and color indices for that cluster. For values of (V-R)(C) and (R-I)(C) published previously by Taylor & Joner, the tests reveal no detectable scale-factor problems. In addition, the tests show that possible zero-point corrections to the published data can be no larger than a few millimagnitudes. These test results indicate that future studies requiring precision photometry for Hyades stars would be well served by selecting data samples from sources as close as possible to the native Cousins system. Tests of B-V photometry published by Johnson & Knuckles reveal a zero-point ambiguity of approximately 8 mmag in the new data that will require further measurements to resolve.