SIMBAD references

We present the results of recent long-term BVR_CI_Cphotometric 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 BVR_CI_Clight 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 yr light curve that we then analyzed to obtain the pulsation spectrum of W Vir. We find a best-fit period P₀=17.27134 days; along with this period and the integer-ratio harmonics P₀/2 through P₀/5 inclusive, we clearly detect two additional periods, P₁ and P_low, that are close to but not exactly 2P₀/3 and 2P₀, respectively. The former, P₁=11.52562 days, we interpret to be the first overtone mode; the latter, P_low=34.59760 days, is close to the beat period of (P^–1₁-P^–1₀)^–1, as well as to the value of 2P₀. 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=2,452,758.172+17.27134E. We compiled an (O-C) diagram spanning 75 yr from 1932 to 2007 using a variety of published photometric data and visual observations from the American Association of Variable Star Observers and derived a period-change term for the ephemeris equal to -9.9x10^–7E², indicating a period decrease.