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Isotope ratios have recently been measured in the atmospheres ofdirectly-imaged and transiting exoplanets from ground-based observations. Thearrival of JWST allows us to characterise exoplanetary atmospheres in furtherdetail and opens up wavelengths inaccessible from the ground. In this work weconstrain the carbon and oxygen isotopes $^{13}$C, $^{18}$O and $^{17}$O fromCO in the atmosphere of the directly-imaged companion VHS 1256 b throughretrievals of the $\sim$4.1-5.3 $\mu$m NIRSpec G395H/F290LP observations fromthe early release science programme (ERS 1386). We detect and constrain$^{13}$C$^{16}$O, $^{12}$C$^{18}$O and $^{12}$C$^{17}$O at 32, 16 and10$\sigma$ confidence respectively, thanks to the very high signal-to-noiseobservations. We find the ratio of abundances are more precisely constrainedthan their absolute values, with $\mathrm{^{12}C/^{13}C=62^{+2}_{-2}}$, inbetween previous measurements for companions ($\sim$30) and isolated browndwarfs ($\sim$100). The oxygen isotope ratios are $\mathrm{^{16}O/^{18}O=425^{+33}_{-28}}$ and $\mathrm{^{16}O/^{17}O=1010^{+120}_{-100}}$. All of theratios are lower than the local inter-stellar medium and Solar System,suggesting that abundances of the more minor isotopes are enhanced compared tothe primary. This could be driven by isotope fractionation in protoplanetarydisks, which can potentially alter the carbon and oxygen ratios through isotopeselective photodissociation, gas/ice partitioning and isotopic exchangereactions. In addition to CO, we constrain $^{1}$H$_2$$^{16}$O and$^{12}$C$^{16}$O$_2$ (the primary isotopologues of both species), but find onlyupper limits on $^{12}$C$^1$H$_4$ and $^{14}$N$^{1}$H$_3$. This work highlightsthe power of JWST to constrain isotopes in exoplanet atmospheres, with greatpromise in determining formation histories in the future.

JWST measurements of $^{13}$C, $^{18}$O and $^{17}$O in the atmosphere of super-Jupiter VHS 1256 b