Reclassification of 2MASS J20295701+4109538 from B0 to O9.5. The target spectrum, obtained with OSIRIS+@GTC at R​ ∼ ​2500, is shown in dark blue. Each panel displays the diagnostic lines used to distinguish O- and early B-type stars, following the criteria of Sota et al. (2011). For comparison, the O- and B-type spectra from the GOSSS catalog (Maíz Apellániz et al. 2016) are included: the O9.5 V standard AE Aur (in green) and the B0 V standard τ Sco (in red).