Rotational periods of four roAp stars

Forty-five new measurements of the mean longitudinal magnetic fields and mean equivalent widths of 4 roAp stars have obtained using the MuSiCoS spectropolarimeter at Pic du Midi observatory. These new high-precision data have been combined with archival measurements in order to constrain the rotational periods of HD 12098, HD 24712 = HR 1217, HD 122970 and HD 176232 = 10 Aql. We report a revised rotational period for HD 24712 ($P_{\rm rot} = 12.45877$ ± 0.00016 d, crucial for interpretation of upcoming MOST observations of this star), new rotational periods for HD 12098 and HD 122970 ($P_{\rm rot}=5.460$ ± 0.001 d and $P_{\rm rot}=3.877$ ± 0.001 d, respectively) and evidence for an extremely long period for HD 176232.