V-band light curve of EX Lup. Filled and empty circles are V-band magnitudes of EX Lup from the All Sky Automated Survey (ASAS-3) and the American Association of Variable Star Observers (AAVSO), respectively. The horizontal dashed line is the pre-outburst optical level. The right y-axis gives the optical flux in pre-outburst flux units. The grey stripe at JD ≃ 2454690 identifies the XMM-Newtonobserving period in August 2008.