The study of hot corinos in Solar-like protostars has been so far mostly limited to the Class 0 phase , hampering our understanding of their origin and evolution .
In addition , recent evidence suggests that planet formation starts already during Class I phase , which , therefore , represents a crucial step in the future planetary system chemical composition .
Hence , the study of hot corinos in Class I protostars has become of paramount importance .
Here we report the discovery of a hot corino towards the prototypical Class I protostar L1551 IRS5 , obtained within the ALMA Large Program FAUST .
We detected several lines from methanol and its isopotologues ( ^ { 13 } CH _ { 3 } OH and CH _ { 2 } DOH ) , methyl formate and ethanol .
Lines are bright toward the north component of the IRS5 binary system , and a possible second hot corino may be associated with the south component .
The methanol lines non-LTE analysis constrains the gas temperature ( \sim 100 K ) , density ( \geq 1.5 \times 10 ^ { 8 } cm ^ { -3 } ) , and emitting size ( \sim 10 au in radius ) .
All CH _ { 3 } OH and ^ { 13 } CH _ { 3 } OH lines are optically thick , preventing a reliable measure of the deuteration .
The methyl formate and ethanol relative abundances are compatible with those measured in Class 0 hot corinos .
Thus , based on the present work , little chemical evolution from Class 0 to I hot corinos occurs .