Context : The transient X-ray source XMMU J004215.8+411924 within M31 was found to be in outburst again in the 2010 May 27 Chandra observation .
We present results from our four Chandra and seven Swift observations that covered this outburst .

Aims : X-ray transient behaviour is generally caused by one of two things : mass accretion from a high mass companion during some restricted phase range in the orbital cycle , or disc instability in a low mass system .
We aim to exploit Einstein , HST , Chandra and Swift observations to determine the nature of XMMU J004215.8+411924 .

Methods : We model the 2010 May spectrum , and use the results to convert from intensity to counts in the fainter Chandra observations , as well as the Swift observations ; these data are used to create a lightcurve .
We also estimate the flux in the 1979 January 13 Einstein observation .
Additionally , we search for an optical counterpart in HST data .

Results : Our best X-ray positions from the 2006 and 2010 outbursts are 0.3 ^ { \prime \prime } apart , and 1.6 ^ { \prime \prime } from the Einstein source ; these outbursts are likely to come from the same star system .
We see no evidence for an optical counterpart with m _ { B } \la 25.5 ; this new limit is 3.5 magnitudes fainter than the existing one .
Furthermore , we see no V band counterpart with m _ { V } \la 26 .
The local absorption is \sim 7 times higher than the Galactic line-of-sight , and provides \sim 2 magnitudes of extinction in the V band .
Hence M _ { V } \ga - 0.5 .
Fits to the X-ray emission spectrum suggest a black hole primary .

Conclusions : We find that XMMU J004215.8+411924 is most likely to be a transient LMXB , rather than a HMXB as originaly proposed .
The nature of the primary is unclear , although we argue that a black hole is likely .