Herschel observations of planetary nebulae in the MESS program First detection of OH + emission in a planetary nebula P.A.M. van Hoof 1 , M. Etxaluze 2 , G.C. Van de Steene 1 , J. Cernicharo 2 , K.M. Exter 3 , J.R. Goicoechea 2 , M.J. Barlow 4 , B.M. Swinyard 5,4 , T. Ueta 6 , T.L. Lim 5 , M.A.T. Groenewegen 1 , E.T. Polehampton 5,7 , and the MESS consortium 1 Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels, Belgium, 2 Departamento de astrofísica, Centro de Astrobiología, CSIC-INTA. Torrejón de Ardoz, 28850 Madrid, Spain 3 Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium, 4 Dept. of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK, 5 Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK 6 Dept. Of Physics and Astronomy, University of Denver, Mail Stop 6900, Denver, CO 80208, USA 7 Dept . of Physics, University of Lethbridge, Lethbridge, Alberta, T1J 1B1, Canada