  We have grown La$_{0.3}$Sr$_{0.7}$CoO$_{3-{\delta}}$ thin films by pulsed
laser deposition on (LaAlO$_3$)$_{0.3}$(Sr$_2$TaAlO$_3$)$_{0.7}$ substrates
with and without a protective LaAlO$_3$ capping layer and investigated their
structural and magnetic properties. We have observed that, in the uncapped
films, the Curie temperature strongly decreased after annealing in helium
atmosphere, and it significantly decreased even in samples stored for several
weeks at room temperature. The decrease of the Currie temperature is caused by
an increase of the concentration of oxygen vacancies, {\delta}. However, we
show that already a 3 nm thin LaAlO$_3$ capping layer can essentially conserve
{\delta} at room temperature, and it considerably slows down the formation of
oxygen vacancies at elevated temperatures.
