We used point-spread function fitting and a differential ensemble determined from a robust median statistic (RoMS) to examine stars in six open clusters in a search for delta Scuti variables. In the search for new variable stars among hundreds or thousands of stars, the RoMS is proved more effective for finding low-amplitude variables than the traditional error-curve approach. This high-precision differential approach was applied to the open clusters NGC 225, NGC 559, NGC 6811, NGC 6940, NGC 7142, and NGC 7160. Thirteen variables, 29 suspected variables, and 65 potential variables were found, and time-series data of the variables are presented. Among the 13 variables we found nine new delta Scuti variables.

Context. BL Cam is an extreme metal-deficient field high-amplitude SX Phe-type variable where a very complex frequency spectrum is detected, with a number of independent nonradial modes excited, unusual among the high-amplitude pulsators in the Lower Classical Instability Strip. Aims. An extensive and detailed study has been carried out to investigate the pulsational content and properties of this object. Methods. The analysis is based on 283 h of CCD observations obtained in the Johnson V filter, during a long multisite photometric campaign carried out along the Northern autumn-winter of 2005-2006. Additionally, multicolour BI photometry was also collected to study the phase shifts and amplitude ratios, between light curves obtained in different filters, for modal discrimination of the main excited modes. Results. The detailed frequency analysis revealed a very rich and dense pulsational content consisting of 25 significant peaks, 22 of them corresponding to independent modes: one is the already known main periodicity f(0) = 25.5765 cd(-1) (Delta V = 153 mmag) and the other 21 are excited modes showing very small amplitudes. Some additional periodicities are probably still remaining in the residuals. This represents the most complex spectrum ever detected in a high-amplitude pulsator of this type. The majority of the secondary modes suspected from earlier works are confirmed here and, additionally, a large number of new peaks are detected. The amplitude of the main periodicity f(0) seems to be stable during decades, but the majority of the secondary modes show strong amplitude changes from one epoch to another. The suspected fundamental radial nature of the main periodicity of BL Cam is confirmed, while the secondary peak f(1) = 25.2523 cd(-1) is identified as a nonradial mixed mode g(4) with l = 1. The radial double-mode nature, claimed by some authors for the main two frequencies of BL Cam, is not confirmed. Nevertheless, the frequency f(6) = 32.6464 cd(-1) could correspond to the first radial overtone.

We have examined the field around the variable stars QS Geminorum and V367 Geminorum. For the delta Scuti-type variable QS Gem, we determined a revised period of 0.1346116 days, which is close to the originally published period from Hipparcos. For the eclipsing system V367 Gem, we found a period of 0.6992285 days, which is substantially different from the published value. In addition, we report five suspected variable stars in the field: one is a long-period eclipsing system, one a likely gamma Doradus star, and the other three are of unknown type.

We have examined the previously understudied delta Scuti stars V966 Herculis and V1438 Aquilae. We find that V966 Her is a stable pulsator with a refined period of 0.1330302 days with a full Vamplitude of 0.096 mag. We also find that V966 Her has an average radial velocity of + 7.8 km s(-1), a full radial velocity amplitude of 7.6 km s(-1), and a v sin i = 63:8 km s(-1). For V1438 Aql we report a revised Hipparcos period of 0.1612751 days with a full amplitude of 0.056. The average radial velocity is found to be -43 km s(-1), with full amplitude of 9.7 km s(-1), and a v sin i 76:7 km s(-1). Due to some anomalies seen in V1438 Aql we feel that a much larger photometric and spectroscopic campaign is required to determine the true nature of this star.