Received: 6.06.2011; accepted: 17.11.2011
(E-mail for contact: PunanovaAnna@gmail.com)
1. R = 15.10 – 15.60; I = 15.05 – 15.46. MinII (V) = 15.93; MinII (I) = 15.30.
2. I = 16.70 – 17.40. MinII (V) = 18.26.
3. R = 14.33 – 14.53; I = 13.75 – 13.86. Possible period exceeds 20 days.
4. R = 14.77 – 14.88; I = 14.83 – 14.95. The star's period, amplitude, spectral type, HR-diagram location in the instability strip suggest the RR type, but it can also be an eclipsing star of the EW type with a period of 0.58 days.
Our observations were performed between February 2 and March 1, 2010 at the Kourovka astronomical observatory with the Hamilton MASTER II telescope of the MASTER Robotic Net (Lipunov et al. 2010).
Astrometric cross-matching and aperture photometry were based on the IRAF package (Tody 1993). For subsequent reductions and differential photometry of 2194 stars in the 30'x30' field of NGC7142, a special C++ console program was developed. The program executes algorithms described by Everett & Howell (2001). Our photometric uncertainties for 11–15 mag stars are within 0.05 mag in all bands. We transformed our instrumental magnitudes to the Johnson–Cousins VRI system using CCD photometry of our field from Crinklaw & Talbert (1991), Magakian et al. (2004) and Hartigan & Lada (1985). Four variable stars were found using robust median statistic (Rose & Hintz 2007).
Differential photometry was performed for all suspected variable stars. For this aim, three comparison stars were used for each variable. The angular distances between variables and their comparison stars were within 30", with magnitude differences less than 0.5 mag. For comparison stars, the rms scatter was from 0.01 to 0.06 mag. The periods were determined using the WinEfk software package developed by Dr. V.P. Goranskij for Windows environment; the Lafler–Kinman method was applied.
This study was financially supported by the State Agency for Science and Innovation of the Ministry of Education and Science of the Russian Federation (state contract No. 02.740.11.0249).
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Hartigan, P., Lada, C.J., 1985, Astrophys. J. Suppl., 59, 383
Lipunov, V., Kornilov, V., Gorbovskoy, E., et al., 2010, Advances in Astronomy, article id. 349171
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