O–C diagram of the main pulsation mode of V391 Peg when using monthly runs (each point represents the data collected within one month). Upper panels: fit of the O–C data with a parabola (long-term variation, blue continuous line) plus a sine wave (“planetary component”, red dashed line) and planetary component alone after subtracting the long-term component. This solution gives satisfactory results only up to the end of 2008, and the fit was made considering only the data up to 2009.0. Lower panels: same as upper panels, but using two sinusoids. In this case, the fit was made using all the data, but a reasonable fit is obtained only up to ~2010, indicating that two components are simply not enough to obtain a reasonable fit of all the data. When we compare the planetary component alone in the period 2000–2009.0, the fit is better when we use parabola + sine wave (χ² = 762) with respect to the double sine wave (χ² = 1267); for comparison, a straight line would give χ² = 1376.