Allowing for TTVs

In its default mode, TransitFit assumes that there are no TTVs and fits a single \(t_0\) and :math: P that are assumed applicable to all transits. In the situation where TTVs are present, TransitFit can fit \(t_0\) to each individual epoch, allowing then for O-C investigations.

In order to do this, set allow_TTV=True in the arguments of run_retrieval(). Note: TransitFit cannot automatically detect if there are TTVs present in the data. You must explicitly enable this mode.

When allow_TTV=True, TransitFit cannot fit for the period value, and this must be set as a fixed value in the data input file. In order to be consistent, we recommend that any investigation into TTVs in a system should have 2 stages:

1. Run TransitFit with allow_TTV=False.

2. Run TransitFit with allow_TTV=True, fixing the period at the best fit value from the first step.