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Context. The continuum is understood to contain a large portion of the energy emitted by a solar flare. The optical continuum, known as white light (WL), is particularly relevant since it may be observed by ground-based instruments. Aims. We measured the WL enhancements short- and longward of the Paschen jump in order to gain insights into the possible mechanism(s) behind the creation of these increases in our two case studies. Methods. We took measurements from the Swedish 1-m Solar Telescope of the pseudo-continuum around the Ca II 8542 Å line as well as the true continuum around the KI7699 Å and the FeI6173 Å line, providing us with observations on both sides of the Paschen jump. Results. We observe WL enhancements of over 40% against the dark (pen-) umbral background in both flares. No WL excess is detectable against the granulation outside the sunspots. The WL excess in flare 1 is co-temporal with the derivative of the GOES soft X-ray and hard X-ray (HXR) measurements from the Advanced Space-based Solar Observatory (ASO-S), and the flare is compatible with the Neupert effect. For the second flare, a preceding smaller flare may be the cause of the temporal discrepancy. Signatures of chromospheric evaporation and condensation are found in the WL area for both flares. The ratio of intensities blueward and red-ward of the Paschen jump (i.e., the Paschen ratio) in flare 1 is below one for most WL pixels. This is in disagreement with the accepted WL formation mechanisms, which are both of photospheric and chromospheric origin. We believe this is a consequence of the Ca II 8542 Å pseudo-continuum being affected by line wing opacity changes. Conclusions. The co-temporality of WL and HXR enhancements suggests that the WL emission enhancements in flare 1 (and parts of flare 2) are a result of direct electron precipitation. We conclude that more reliable continuum measurements free of any nearby line influence are necessary in order to obtain conclusive evidence for the formation mechanism(s) behind optical continuum enhancements from such analysis as presented in this work.

Measuring the orbital periods and the beat periods between the spin and the orbital periods of magnetic cataclysmic variables is very important to understand their physical properties and evolutionary states. Based on long-term, high-precision optical data from TESS, this study performs a detailed time-variability analysis of three sources, IGR J17040-4305, Swift J170800-402551.8, and AX J1832.3-0840, and reanalyzes the XMM-Newton and NuSTAR observations. The power spectrum of the XMM-Newton data shows that the white dwarf spin period in IGR J17040-4305 is 957.44 ± 1.52 s. The TESS data further reveal its candidate orbital period and beat period to be 4.0821 ± 0.0008 hr and 1024.527 ± 0.011 s, respectively. For Swift J170800-402551.8, the NuSTAR FPMA data yield a candidate orbital period of 3.424 ± 0.059 hr, while the TESS data give a spin period of 783.64 ± 0.04 s and a candidate orbital period of 3.470 ± 0.002 hr. For AX J1832.3-0840, the TESS data measure the spin period, candidate orbital period, and beat period to be 1550.25 ± 0.09 s, 1.3838 ± 0.0003 hr, and 2251.11 ± 0.14 s, respectively. Based on the precise measurements of the candidate orbital periods of these three sources, their classification as intermediate polars is strongly supported. Furthermore, using the X-ray spectra from XMM-Newton and NuSTAR, the white dwarf masses of IGR J17040-4305 and Swift J170800-402551.8 are approximately estimated to be 0.774 ± 0.025M<SUB>⊙ </SUB>and 1.141 ± 0.062M<SUB>⊙</SUB>, respectively.

The majority of massive stars are born in close binary systems. As stars expand when they age, mass transfer or even a merger with their companion is inevitable. However, most binary interaction products appear as single stars, such that the main evidence of their exciting past is lost. In a comprehensive grid of detailed massive binary evolution models we find systematic trends in chemical surface abundances that allow identifying the past mass gainers. We develop an analytic framework which is independent of specific evolutionary models, to constrain the amount and composition of the accreted material from their observed surface abundances. This yields tight constraints on the uncertain mass transfer physics in massive binary stars and allows us to reconstruct the past evolutionary history of the progenitor binary system. This method, which is shown to also constrain binary mergers (for example, SN 1987A), is applied to some of the best-studied OB stars so far. For γ Columbae, suggested to be an envelope-stripped star, we show that it is a mass gainer instead, whose companion star likely formed a stripped-envelope supernova. Our results highlight surface abundance measurements as a powerful tool to improve our understanding of massive binary systems evolving towards supernovae and compact object binaries.