On the double heat capacity peak of oxygen solid-to-solid transition near 23.8 K

•Two knwon solid–solid transitions for Oxygen: the α -β at about 23.9 K and the β - γ at about 43.9 K.•Now a new transition is found, a mere 70 mK below the αϕ- β transition.•Using a high quality calorimeter as used at National Metrology Institutes for the realization of the cryogenic fixed points of the ITS90.•The transition area needed very slow scanningly, up to 20 h. Faster scanning did non show the new transition.