Effects of the spin-orbit interaction on the optical properties of ReS2 and ReSe2

${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ are less frequently studied transition metal dichalcogenides. They appear in the $1{T}^{\ensuremath{'}}$ phase with a significantly reduced symmetry compared with, for example, ${\mathrm{MoS}}_{2}$, while inversion symmetry is preserved. Several broad peaks have been observed in previous experimental measurements. The interpretation of their physical origin and properties is a challenging task without the help of theoretical insights. Here, we employ ab initio $GW$-Bethe-Salpeter equation calculations to investigate the optical properties of neutral and charged monolayers. We present a detailed analysis of the low-energy excitons and trions. In agreement with experiment, we find that the neutral excitons are strongly bound. In contrast, only a tiny trion binding energy of 10 meV is found. By artificially manipulating the spin-orbit coupling (SOC), we demonstrate its importance for the internal structure of the transitions. Interestingly, we find two optically active low-energy excitons. Both stem from transitions between the same valence and conduction bands. In the absence of SOC, only one peak (singlet) can be bright. Therefore, additional peaks are a direct manifestation of SOC on the optical properties.