2018A&A...611A..85S

V391 Peg (alias HS 2201+2610) is a subdwarf B (sdB) pulsating star that shows both p- and g-modes. By studying the arrival times of the p-mode maxima and minima through the O-C method, in a previous article the presence of a planet was inferred with an orbital period of 3.2 years and a minimum mass of 3.2M_Jup. Here we present an updated O-C analysis using a larger data set of 1066 h of photometric time series (∼2.5x larger in terms of the number of data points), which covers the period between 1999 and 2012 (compared with 1999-2006 of the previous analysis). Up to the end of 2008, the new O-C diagram of the main pulsation frequency (f₁) is compatible with (and improves) the previous two-component solution representing the long-term variation of the pulsation period (parabolic component) and the giant planet (sine wave component). Since 2009, the O-C trend of f₁ changes, and the time derivative of the pulsation period (dp/dt) passes from positive to negative; the reason of this change of regime is not clear and could be related to nonlinear interactions between different pulsation modes. With the new data, the O-C diagram of the secondary pulsation frequency (f₂) continues to show two components (parabola and sine wave), like in the previous analysis. Various solutions are proposed to fit the O-C diagrams of f₁ and f₂, but in all of them, the sinusoidal components of f₁ and f₂ differ or at least agree less well than before. The nice agreement found previously was a coincidence due to various small effects that are carefully analyzed. Now, with a larger dataset, the presence of a planet is more uncertain and would require confirmation with an independent method. The new data allow us to improve the measurement of dp/dt for f₁ and f₂: using only the data up to the end of 2008, we obtain (dp/dt)₁=(1.34±0.04)x10^–12 and (dp/dt)_2=(1.62±0.22)x10^–12. The long-term variation of the two main pulsation periods (and the change of sign of (dp/dt)₁ is visible also in direct measurements made over several years. The absence of peaks near f₁ in the Fourier transform and the secondary peak close to f₂ confirm a previous identification as l=0 and l=1, respectively, and suggest a stellar rotation period of about 40 days. The new data allow constraining the main g-mode pulsation periods of the star.