GRB 210210A: CAHA detection and likely jet break

We observed the afterglow (Laha et al., GCN #29444, Lipunov et al., GCN #29447, de Ugarte Postigo et al., GCN #29450, Breeveld & Laha, GCN #29457, Strausbaugh & Cucchiara, GCN #29459, Jelinek et al., GCN #29469) of the very soft GRB 210210A (Laha et al., GCN #29444) with CAFOS mounted on the 2.2m Calar Alto telescope (Almeria, Spain). We obtained one 180 s image each in BVRc and 3 x 90 s images in Ic. We detect the optical afterglow faintly in BVRc. Using the same comparison star used by de Ugarte Postigo et al., and transforming to Rc following the equations of Lupton (2005), then transforming back to AB magnitudes, we derive: Rc = 21.89 +/- 0.21 mag at 1.167362 days after the trigger. Using the R-band magnitudes from the GCNs listed above (FRAM, Jelinek et al.; GTC, de Ugarte Postigo et al.; LCO, Strausbaugh & Cucchiara), we find an unbroken decay with alpha_1 = 1.05 until 0.41 days post-trigger. However, our detection lies 0.9 magnitudes below the extrapolation. Fixing the break time to 0.41 days, we find a lower limit on the post-break decay alpha_2 > 1.93. We note the X-ray afterglow also shows a potential jet break, with alpha_2 > 3, however, more observations are needed.