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TIC137608661/TYC4544-2658-1/FBS0938+788 is a new sdBV+dM reflection-effectbinary discovered by the TESS space mission with an orbital period of 7.21hours. In addition to the orbital frequency and its harmonics, the Fouriertransform of TIC137608661 shows many g-mode pulsation frequencies from the sdBstar. The amplitude spectrum is particularly simple to interpret as weimmediately see several rotational triplets of equally spaced frequencies. Thecentral frequencies of these triplets are equally spaced in period with a meanperiod spacing of 270.12 s, corresponding to consecutive l=1 modes. From themean frequency spacing of 1.25 {\mu}Hz we derive a rotation period of 4.6 daysin the deep layers of the sdB star, significantly longer than the orbitalperiod. Among the handful of sdB+dM binaries for which the sdB rotation wasmeasured through asteroseismology, TIC137608661 is the non-synchronized systemwith both the shortest orbital period and the shortest core rotation period.Only NY Vir has a shorter orbital period but it is synchronized. From aspectroscopic follow-up of TIC137608661 we measure the radial velocities of thesdB star, determine its atmospheric parameters, and estimate the rotation rateat the surface of the star. This measurement allows us to exclude synchronizedrotation also in the outer layers and suggests a differential rotation, withthe surface rotating faster than the core, as found in few other similarsystems. Furthermore, an analysis of the spectral energy distribution ofTIC137608661, together with a comparison between sdB pulsation properties andasteroseismic models, gives us further elements to constrain the system.

monthly notices of the royal astronomical society

Filling the gap between synchronized and non-synchronized sdBs in  short-period sdBV+dM binaries with TESS: TIC 137608661, a new system with a  well defined rotational splitting