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The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) inMarch-April 2013 and January 2015, combined with the improved observationalcapabilities of submillimeter facilities, offered an opportunity to carry outsensitive compositional and isotopic studies of the volatiles in their coma. Weobserved comet Lovejoy with the IRAM 30m telescope between 13 and 26 January2015, and with the Odin submillimeter space observatory on 29 January - 3February 2015. We detected 22 molecules and several isotopologues. TheH$_2^{16}$O and H$_2^{18}$O production rates measured with Odin follow aperiodic pattern with a period of 0.94 days and an amplitude of ~25%. Theinferred isotope ratios in comet Lovejoy are $^{16}$O/$^{18}$O = 499 $\pm$ 24and D/H = 1.4 $\pm$ 0.4 $\times 10^{-4}$ in water, $^{32}$S/$^{34}$S = 24.7$\pm$ 3.5 in CS, all compatible with terrestrial values. The ratio$^{12}$C/$^{13}$C = 109 $\pm$ 14 in HCN is marginally higher than terrestrialand $^{14}$N/$^{15}$N = 145 $\pm$ 12 in HCN is half the Earth ratio. Severalupper limits for D/H or 12C/13C in other molecules are reported. From ourobservation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratioin an Oort Cloud comet that is not larger than the terrestrial value. On theother hand, the observation of the same HDO line in the other Oort-cloud comet,C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previousmeasurements of D/H in cometary water, this illustrates that a diversity in theD/H ratio and in the chemical composition, is present even within the samedynamical group of comets, suggesting that current dynamical groups containcomets formed at very different places or times in the early solar system.

Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy)