The Shortest Modulation Period Blazhko RR Lyrae Star: SS Cancri

Extended BV(RI)c CCD observations of SS Cnc, a short period RRab star arepresented. Nearly 1400 data points in each band have been obtained spanningover 79 days during the spring of 2005. The star exhibits light curvemodulation, the so called Blazhko effect with small amplitude (B maximumbrightness varies 0.1 mag) and with the shortest modulation period (5.309 d)ever observed. In the Fourier spectrum of the V light curve the pulsationfrequency components are detected up to the 24th harmonic order, and modulationside lobe frequencies with significantly asymmetric amplitudes are seen up tothe 15th and 9th orders for the lower and higher frequency components,respectively. Detailed comparison of the modulation behavior of SS Cnc and RRGem, the two recently discovered small amplitude, short modulation periodBlazhko stars is presented. The modulation frequency (f_m) appears in theFourier spectrum of both stars with similar amplitude. We also demonstrate thatthe modulation frequencies have basically different properties as the pulsationand modulation side lobe frequencies have, indicating that the physics behindthese frequency components are not the same. The discovery of small amplitudemodulations of RRab stars cautions that the large photometric surveys (MACHO,OGLE) may seriously underestimate the number of modulated RR Lyrae stars.Comment: 23 pages, 9 figures, 2 tables. To appear in A