EP250108a: further NOT optical observations

We observed the possible optical counterpart (Eyles-Ferris, GCN 38878; Zhu et al., GCN 38885) of the Einstein Probe transient EP250108a (Li et al., GCN 38861), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out in the griz filters and began on Jan 10.88 UT (2.36 days after the trigger). Conditions were not ideal, with a nearby full Moon and mediocre seeing (1.7-2"). The counterpart is detected in all filters, and shows a remarkably blue SED. Comparison of the new data with those taken on the night of Jan 9 (Zhu et al., GCN 38885) shows moderate or no change in brightness in the g band, and a fading in the redder filters. The spectral slope can be roughly fitted with a power-law F_nu propto nu^2, consistent with a hot black-body. The bright, blue optical spectral shape and the lack of X-ray emission (Li et al., GCN 38888) of this transient are very unusual among GRB and FXT afterglows, as already noted by Zhu et al. (GCN 38885). If associated with EP250108a, the transient is revealing an uncommon emission component, or could be due to a different progenitor. We acknowledge expert support from the NOT observers Andrea Henderson de la Fuente and Samuel Grund Sorensen.