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Classical novae (CNe) represent a major class of supersoft X-ray sources(SSSs) in the central region of our neighbouring galaxy M 31. Significantlydifferent SSS properties of CNe in the M 31 bulge and disk were indicated byrecent X-ray population studies, which however considered only a small numberof disk novae. We initiated a target of opportunity (ToO) program withXMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve thedatabase for further population studies. We analysed two XMM-Newton ToOobservations triggered in Aug 2011 and Jan 2012, respectively, and extractedX-ray spectra and light curves. We report the discovery of an X-ray counterpartto the M 31 disk nova M31N 2008-05d. The X-ray spectrum of the object allows usto classify it as a SSS parametrised by a blackbody temperature of 32+/-6 eV.More than three years after the nova outburst, the X-ray light curve of the SSSexhibits irregular, broad dip features. These dips affect primarily the verysoft part of the X-ray spectrum, which might indicate absorption effects.Dipping SSS light curves are rarely observed in M 31 novae. As well asproviding an unparalleled statistical sample, the M 31 population of novae withSSS counterparts produces frequent discoveries of unusual objects, therebyunderlining the importance of regular monitoring.

M31N 2008-05d: A M 31 disk nova with a dipping supersoft X-ray light curve