At group meeting at the end of the day, Ruth Angus (Oxford) showed her attempts to measure reliable rotation periods for stars using photometric variability. Both because of the short lives of sunspots (and other surface features), and because of differential rotation, the variability induced by rotation is not periodic and certainly not harmonic. Nonetheless, it shows up in the autocorrelation function as a negative autocorrelation at half-period lags and a positive correlation at full period. That said, the autocorrelation functions are noisy and they are point estimates. She is trying to move to a probabilistic framework using Gaussian Processes. She showed some early results and we offered consulting.
In the same meeting, Soichiro Hattori (NYUAD) showed some first results for planet search in the Kepler field, using a trivial "box model" and simple squared error objective. It looks like it works: When he scans through parameters, he gets minima in the right places. His next steps are to switch to a more sophisticated noise model (likelihood function) and then make it very fast.