Marie-Louise Langlais instructs RCO students at the console in the the church of the Oratoire du Louvre, Paris   This year's RCO study trip was focused around organ music of the French Romantic period, in particular the music of Jean Langlais and Oliver Messiaen.  Seven playing students and five observers, led by RCO tutor Richard Brasier, visited four churches in the center of Paris over several days in June, to play on instruments by the builder Aristide Cavaillé-Coll.  They were also invited to the former apartment of Maurice Duruflé, which still contains his house organ.  Rachel…