Using HCO+ isotopologues as tracers of gas depletion in protoplanetary disk gaps Grigorii V. Smirnov-Pinchukov, Dmitry A. Semenov, Vitaly V. Akimkin and Thomas Henning A&A, 644 (2020) A4 Published online: 24 November 2020 DOI: 10.1051/0004-6361/202038572