### Abstract

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 3.2 M_{Jup}. Here we present an updated O-C analysis using a larger data set of 1066 h of photometric time series ( 2.5× 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 (f_{1}) 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 f_{1} changes, and the time derivative of the pulsation period (p^{.}) 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 (f_{2}) 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 f_{1} and f_{2}, but in all of them, the sinusoidal components of f_{1} and f_{2} 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 p^{.} for f_{1} and f_{2}: using only the data up to the end of 2008, we obtain p^{.}_{1} = (1.34 ± 0.04) × 10^{-12} and p^{.}_{2} = (1.62 ± 0.22) × 10^{-12}. The long-term variation of the two main pulsation periods (and the change of sign of p^{.}_{1}) is visible also in direct measurements made over several years. The absence of peaks near f_{1} in the Fourier transform and the secondary peak close to f_{2} 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.

Original language | English |
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Article number | A85 |

Journal | Astronomy and Astrophysics |

Volume | 611 |

DOIs | |

Publication status | Published - márc. 1 2018 |

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### ASJC Scopus subject areas

- Astronomy and Astrophysics
- Space and Planetary Science

### Cite this

*Astronomy and Astrophysics*,

*611*, [A85]. https://doi.org/10.1051/0004-6361/201731473