Multicolor Photometry of the Type II Cepheid Prototype W Virginis

Abstract

We present the results of recent long-term BVRcIc photometric monitoring of the type II Cepheid prototype W Virginis. These new observations, made during the 2006 and 2007 observing season, represent the longest homogeneous, multicolor light curve of W Vir to date. The BVRcIc light and color curves show conclusively that W Vir exhibits modest but detectable cycle-to-cycle variations, the cause of which appears to be multiperiodicity rather than nonlinearity. We combined our V-band data with the five available years of ASAS-3 V-band photometry to obtain a 6.5-year light curve that we then analyzed to obtain the pulsation spectrum of W Vir. We find a best-fit period P(0) = 17.27134 d; along with this period and its integer harmonics, we clearly detect two additional periods, P(1) and P(low), that are close to but not exactly 2P(0)/3 and 2P(0), respectively. The former, P(1) = 11.52562 d, we interpret to be the first overtone mode; the latter, P(low) = 34.59760 d is close to the beat period of P(0) and P(1), as well as to the value of 2P(0). We interpret the previously reported but thus far unconfirmed descriptions of alternating minima as manifestations of this multiperiodicity. Finally, we use the period derived from the V-band light curve to define a new ephemeris: HJD(max) = 2452758.172 + 17.27134E. The resulting (O-C) diagram using 75 years of data from 1932 to 2007 yields a period change term for the ephemeris of -9.9 times 10(-7) E^(2), indicating a period decrease.