Detection of photometric variability in the very low-mass binary VHS J1256-1257AB using TESS and Spitzer

We report the detection of photometric variability in the M7.5 binary star VHS J1256-1257AB using archival data from TESS and Spitzer. The optical and infrared light curves periodically exhibit epochs of quasi-sinusoidal modulation followed by others of stochastic variability, which resembles the beat pattern created by two waves of similar frequencies that interfere with each other. Our two-waves model shows that the components of VHS J1256-1257AB rotate with periods of 2.0782 ± 0.0004 h and 2.1342 ± 0.0003 h. As a result the fluxes of the equal-brightness VHS J1256-1257A and B alternate states of phase and anti-phase, explaining the observed photometric variability in their combined light. This is only the first example of several beat-patterns revealed by TESS in different binaries  (Miles-Páez, 2021, A&A, 651, 7).