We present observations and analysis of 18 stripped-envelope supernovae observed during 2013 -2018. This sample consists of 5 H/He-rich SNe, 6 H-poor/He-rich SNe, 3 narrow lined SNe Ic and 4 broad lined SNe Ic. The peak luminosity and characteristic time-scales of the bolometric light curves are calculated, and the light curves modelled to derive 56 Ni and ejecta masses (M Ni and M ej ). Additionally, the temperature evolution and spectral line velocity-curves of each SN are examined. Analysis of the [O i] line in the nebular phase of eight SNe suggests their progenitors had initial masses < 20 M . The bolometric light curve properties are examined in combination with those of other SE events from the literature. The resulting dataset gives the M ej distribution for 80 SE-SNe, the largest such sample in the literature to date, and shows that SNe Ib have the lowest median M ej , followed by narrow lined SNe Ic, H/He-rich SNe, broad lined SNe Ic, and finally gamma-ray burst SNe. SNe Ic-6/7 show the largest spread of M ej ranging from ∼ 1.2 − 11 M , considerably greater than any other subtype. For all SE-SNe = 2.8 ± 1.5 M which further strengthens the evidence that SE-SNe arise from low mass progenitors which are typically < 5 M at the time of explosion, again suggesting M ZAMS < 25 M . The low and lack of clear bimodality in the distribution implies < 30 M progenitors and that envelope stripping via binary interaction is the dominant evolutionary pathway of these SNe.

Results. Photometry of the 2013 eruption, combined with three previous eruptions, enabled construction of a template lightcurve of a very fast nova (t 2 (V) 4 days). The archival data allowed recovery of the progenitor system in optical and near-UV data, indicating a red-giant secondary with bright accretion disk, or alternatively a system with a sub-giant secondary but dominated by a disk. Conclusions. The eruptions of M 31N 2008-12a, and a number of historic X-ray detections, indicate a unique system with a recurrence timescale of ∼1 yr. This implies the presence of a very high-mass white dwarf and a high accretion rate. The recovered progenitor system is consistent with such an elevated rate of accretion. We encourage additional observations, especially towards the end of 2014.

We present observations of M31LRN 2015 (MASTER OT J004207.99+405501.1), discovered in M31 in 2015 January, and identified as a rare and enigmatic luminous red nova (LRN). Spectroscopic and photometric observations obtained by the Liverpool Telescope showed the LRN becoming extremely red as it faded from its = − ± M 9.4 0.2 V peak. Early spectra showed strong Hα emission that weakened over time as a number of absorption features appeared, including Na I D and Ba II. At later times strong TiO absorption bands were also seen. A search of archival Hubble Space Telescope data revealed a luminous red source to be the likely progenitor system, with pre-outburst Hα emission also detected in ground-based data. The outburst of M31LRN 2015 shows many similarities, both spectroscopically and photometrically, with that of V838 Mon, the best studied LRN. We finally discuss the possible progenitor scenarios.

Context. Another outburst of the recurrent M 31 nova M31N 2008-12a was announced in late November 2013. Optical data suggest an unprecedentedly short recurrence time of approximately one year. Aims. In this Letter we address the X-ray properties of M31N 2008-12a. Methods. We requested Swift monitoring observations shortly after the optical discovery. We estimated source count rates and extracted X-ray spectra from the resulting data. The corresponding ultraviolet (UV) data were also analysed. Results. The nova M31N 2008-12a was clearly detected as a bright supersoft X-ray source (SSS) only six days after the wellconstrained optical discovery. It displayed a short SSS phase of two weeks' duration and an exceptionally hot X-ray spectrum with an effective black-body temperature of ∼97 eV. During the SSS phase the X-ray light curve displayed significant variability that might have been accompanied by spectral variations. The very early X-ray variability was found to be anti-correlated with simultaneous variations in the UV flux. Conclusions. The X-ray properties of M31N 2008-12a coherently point towards a high-mass white dwarf in the nova system. This object might be a promising Type Ia supernova progenitor. We rediscovered additional X-ray detections of M31N 2008-12a that are consistent with our data and increase the number of known nova outbursts to seven. This nova is an exceptional object that merits further attention in the future.

The reported positions of 964 suspected nova eruptions in M31 recorded through the end of calendar year 2013 have been compared in order to identify recurrent nova candidates. To pass the initial screen and qualify as a recurrent nova candidate two or more eruptions were required to be coincident within 0.1 , although this criterion was relaxed to 0.15 for novae discovered on early photographic patrols. A total of 118 eruptions from 51 potential recurrent nova systems satisfied the screening criterion. To determine what fraction of these novae are indeed recurrent the original plates and published images of the relevant eruptions have been carefully compared. This procedure has resulted in the elimination of 27 of the 51 progenitor candidates (61 eruptions) from further consideration as recurrent novae, with another 8 systems (17 eruptions) deemed unlikely to be recurrent. Of the remaining 16 systems, 12 candidates (32 eruptions) were judged to be recurrent novae, with an additional 4 systems (8 eruptions) being possibly recurrent. It is estimated that ∼4% of the nova eruptions seen in M31 over the past century are associated with recurrent novae. A Monte Carlo analysis shows that the discovery efficiency for recurrent novae may be as low as 10%