  Isotopic ratios in comets are critical to understanding the origin of
cometary material and the physical and chemical conditions in the early solar
nebula. Comet C/2015 ER61 (PANSTARRS) underwent an outburst with a total
brightness increase of 2 magnitudes on the night of 2017 April 4. The sharp
increase in brightness offered a rare opportunity to measure the isotopic
ratios of the light elements in the coma of this comet. We obtained two
high-resolution spectra of C/2015 ER61 with UVES/VLT on the nights of 2017
April 13 and 17. At the time of our observations, the comet was fading
gradually following the outburst. We measured the nitrogen and carbon isotopic
ratios from the CN violet (0,0) band and found that $^{12}$C/$^{13}$C=100 $\pm$
15, $^{14}$N/$^{15}$N=130 $\pm$ 15. In addition, we determined the
$^{14}$N/$^{15}$N ratio from four pairs of NH$_2$ isotopolog lines and measured
$^{14}$N/$^{15}$N=140 $\pm$ 28. The measured isotopic ratios of C/2015 ER61 do
not deviate significantly from those of other comets.
