Correlated semi-annual variability in angularly nearby sources. Upper panel: Light curves of the strong type IIb source 0922+005 (black dots) and of 0837+035 (green dots). The NDCF between the de-trended light curves is shown in the lower panel; it reveals a 0.5 yr periodic modulation (magenta line) with a time delay that is consistent with the difference in the solar elongation minima of the objects.