M31N 2008-12a is a most remarkable star in the great Andromeda Galaxy, M31. It undergoes a nova outburst every 11 months on average, much more frequently than any other known recurrent nova. At its brightest it reaches 18th magnitude, putting it within range of amateur astronomers with moderately large telescopes (apertures of 20cm or above) and CCD cameras. The brightness in its quiescent state is unknown but undoubtedly fainter than 23rd magnitude. According to theoretical models this star is likely to explode as a supernova Real Soon Now, meaning within 20 thousand years. When that happens it will become very bright indeed, possibly becoming visible to the naked eye over here, around 2.5 million light years away. A good starting point for researching what is known about M31N 2008-12a is this paper. My attempt at reproducing the published finder chart appears below.
Stop Press: The expected 2019 eruption appears to have been detected on 2019-11-06. This is the ATel