Follow-up observations of X-ray emitting hot subdwarf stars: the compact He-poor sdO star Feige 34 N. La Palombara, S. Mereghetti, P. Esposito and A. Tiengo A&A, 626 (2019) A29 Published online: 06 June 2019 DOI: 10.1051/0004-6361/201935339