Fortnightly fluctuations in the O-C diagram of CS 1246

Dominated by a single, large-amplitude pulsation mode, the rapidly pulsating hot subdwarf B star CS 1246 is a prime candidate for a long-term O-C diagram study. We collected nearly 400 h of photometry with the 0.41-m Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes over a time-span of 14 months to begin looking for secular variations in the pulse timings. Interestingly, the O-C diagram is dominated by a strong sinusoidal pattern with a period of 14.1 d and an amplitude of 10.7 light-seconds. Underneath this sine wave is a secular trend implying a decrease in the 371.7-s pulsational period of ?, which we attribute to the evolution of the star through the Hertzsprung-Russell diagram. The sinusoidal variation could be produced by the presence of a low-mass companion, with m sin i≃ 0.12 M_⊙, orbiting the subdwarf B star at a distance of 20 R_⊙. An analysis of the combined light curve reveals the presence of a low-amplitude first harmonic to the main pulsation mode. Based on observations at the SOAR Telescope, a collaboration between CPNq-Brazil, NOAO, UNC and MSU, and on observations collected at the PROMPT array via SKYNET.