Superorbital Modulation in the High-Mass X-ray Binary 4U 1538−52, and Possible Modulation in IGR J16393−4643

This work makes use of optical observations from the LCOGT network, and we thank the XB-NEWS team and especially Dan Bramich and Dave Russell for providing the photometric analysis of these and advice. This paper made use of Swift/BAT transient monitor results provided by the Swift/BAT team and Fermi GBM results provided by the Fermi GBM team. We thank Malcolm Coe for productive discussion on optical monitoring and Nazma Islam for useful discussion on superorbital models. We also thank the referee for valuable comments.