Modelling of the spin period evolution of SXP 15.6 based on orbital modulation and intrinsic spin-up. The most probable solution is shown as the dashed line, and the dotted black line shows the Doppler shifts due to orbital modulation alone, without the spin-up due to accretion. A family of orbits drawn from the sample of the posterior distribution of parameters from the Bayesian modelling is shown as red lines. The shaded region is the 99% quantile of the models. Although Swift/XRT data present hits of periodic modulation, the uncertainties are quite high and cannot constrain our model. For visualisation purposes, we plot one of these XRT measurements.