088486

During AO 18 our team discovered the presence of two IR bursts lagging thermonuclear Type-I X-ray burstsdoublePoint this allowed us to put the first constraints on the orbital period of the source 4U 1728-34. Moreover comma the profiles of the newly discovered IR bursts showed intriguing .dips. just after the onset of the X-ray bursts. The aim of this proposal is to use the novel method of X-ray-IR burst lag to measure the orbital period of 4U 1728-34 and two other sources comma and to quantify the presence of .dips. in their IR reverberation comma signatures that would indicate burst-induced changes in the geometry of the accretion flow. We therefore ask to perform 4 openPartotal 56ksclosePar XMM pointings to be matched by 4 openPartotal 14hclosePar HAWK-IarrobaVLT observations of 4U 1728-34 comma 4U 1705-44 and 4U 1636-53.