The  main  task  of  the  Beam Energy Scan  (BES)  program  at  RHIC  is  to  scan  the  QCD  phase diagram with heavy-ion  Au+Au  collisions  ( √sNN = 7.7 - 62.4 GeV )  to  find  signatures  for  the  1st-order  phase transition and the critical point.
Femtoscopy analysis allows us to extract information about the size of the emission source. In  particular,  from the non-identical particles correlations,  e.g. pion-kaon femtoscopy, one can obtain information about asymmetry  in emission processes  of pions and kaons.  This asymmetry gives knowledge of which type of  particles  is  emitted first/second  or/and from which area of the source.  
In this talk,  we will present STAR results of pion-kaon femtoscopy analysis at mid-rapidity in Au+Au collisions √sNN = 39 GeV.