CCD Observations and Period Change of the Type ab RR Lyrae Star DV Mon

Abstract

We took a total of 635 B-, V-, and I_c-band CCD frames for the RRab Lyr type variable DVMon, which has a close bright visual companion with a separation of about 1.9'. Observations were made with the 76-cm telescope of the South African Astronomical Observatory (SAAO, South Africa) using SBIG CCD ST-10XME. For the first time, we obtained reliable separate PSF-photometry of both stars and determined their coordinates. We used all available data to construct the O — C diagram spanning a 110-year long time interval, which allowed us to reveal at least three sudden changes of the pulsation period around JD 2438000, 2453500, and 2456500. Our high resolution echelle spectra with the Southern African Large Telescope (SALT) showed that DV Mon belongs to type ab RR Lyrae variables of the Galactic thick disk.