  We report the discovery of a large-amplitude oscillation in the hot subdwarf
B star CS 1246 and present multi-colour photometry and time-resolved
spectroscopy supporting this discovery. We used the 0.41-m PROMPT array to
acquire data in the u', g', r', and i' filters simultaneously over 3
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 hrs to look for line profile variations. Models fits to the spectra
give mean atmospheric values of Teff = 28450 +/- 700 K and log g = 5.46 +/-
0.11 undergoing variations with semi-amplitudes of 507 +/- 55 K and 0.034 +/-
0.009 dex, respectively. We also detect a radial velocity oscillation with an
amplitude of 8.8 +/- 1.1 km/s. The relationship between the angular and
physical radii variations shows 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 +/- 140 pc, R = 0.19 +/- 0.08 Rsun, and M
= 0.39 +/- 0.22 Msun, respectively, using the Baade-Wesselink method.
