| Abstract
| - We report the discovery of a large-amplitude oscillation in the hot subdwarf B star CS 1246 and present multicolour photometry and time-resolved spectroscopy supporting this discovery. We used the 0.41-m Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes array to acquire data in the u′, g′, r′ and i′ filters simultaneously over three consecutive nights in 2009 April. These data reveal a single oscillation mode with a period of 371.707 ± 0.002 s and an amplitude dependent upon wavelength, with a value of 34.5 ± 1.6 mma in the u′ filter. We detected no additional frequencies in any of the light curves. Subsequently, we used the 4.1-m SOAR telescope to acquire a time-series of 248 low-resolution spectra spanning 6 h to look for line profile variations. Models fits to the spectra give mean atmospheric values of Teff= 28 450 ± 700 K and log g= 5.46 ± 0.11 undergoing variations with semi-amplitudes of 507 ± 55 K and 0.034 ± 0.009, respectively. We also detect a radial velocity oscillation with an amplitude of 8.8 ± 1.1 km s−1. The relationship between the angular and physical radii variations shows that the oscillation is consistent with a radial mode. Under the assumption of a radial pulsation, we compute the stellar distance, radius and mass as d=460 ±19090 pc, R= 0.19 ± 0.08 R⊙ and M= 0.39 ±0.300.13 M⊙, respectively, using the Baade-Wesselink method.
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