The Shortest Modulation Period Blazhko RR Lyrae Star: SS Cancri

Extended BV(RI)_C CCD observations of SS Cnc, a short-period RRab star, are presented. Nearly 1400 data points in each band have been obtained, spanning over 79 days during the spring of 2005. The star exhibits light-curve modulation, the so-called Blazhko effect, with small amplitude (B maximum brightness varies by 0.1 mag) and with the shortest modulation period (5.309 days) ever observed. In the Fourier spectrum of the V light curve, the pulsation frequency components are detected up to the 24th harmonic order, and modulation sidelobe frequencies with significantly asymmetric amplitudes are seen up to the 15th and 9th orders for the lower and higher frequency components, respectively. A detailed comparison of the modulation behavior of SS Cnc and RR Gem, two recently discovered small-amplitude, short-modulation-period Blazhko stars, is presented. The modulation frequency (f_m) appears in the Fourier spectrum of both stars with similar amplitude. We also demonstrate that the modulation frequencies have basically different properties from those of the pulsation and modulation sidelobe frequencies, indicating that the physics behind these frequency components is not the same. The discovery of small amplitude modulations of RRab stars cautions that the large photometric surveys (MACHO and OGLE) may seriously underestimate the number of modulated RR Lyrae stars.