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M dwarfs are common host stars to exoplanets but often lack atmosphericabundance measurements. Late-M dwarfs are also good analogs to the youngestsubstellar companions, which share similar $T_{\rm eff}\sim2300-2800~K$. Wepresent atmospheric analyses for the M7.5 companion HIP 55507 B and its K6Vprimary star with Keck/KPIC high-resolution ($R\sim35,000$) $K$ bandspectroscopy. First, by including KPIC relative radial velocities between theprimary and secondary in the orbit fit, we improve the dynamical mass precisionby 60% and find $M_B=88.0_{-3.2}^{+3.4}$ $M_{\rm Jup}$, putting HIP 55507 Babove the stellar-substellar boundary. We also find that HIP 55507 B orbits itsK6V primary star with $a=38^{+4}_{-3}$ AU and $e=0.40\pm0.04$. From atmosphericretrievals of HIP 55507 B, we measure $\rm [C/H]=0.24\pm0.13$, $\rm[O/H]=0.15\pm0.13$, and $\rm C/O=0.67\pm0.04$. Moreover, we strongly detect$\rm ^{13}CO$ ($7.8\sigma$ significance) and tentatively detect $\rm H_2^{18}O$($3.7\sigma$ significance) in companion's atmosphere, and measure $\rm^{12}CO/^{13}CO=98_{-22}^{+28}$ and $\rm H_2^{16}O/H_2^{18}O=240_{-80}^{+145}$after accounting for systematic errors. From a simplified retrieval analysis ofHIP 55507 A, we measure $\rm ^{12}CO/^{13}CO=79_{-16}^{+21}$ and $\rmC^{16}O/C^{18}O=288_{-70}^{+125}$ for the primary star. These resultsdemonstrate that HIP 55507 A and B have consistent $\rm ^{12} C/^{13}C$ and$\rm ^{16}O/^{18}O$ to the $<1\sigma$ level, as expected for a chemicallyhomogeneous binary system. Given the similar flux ratios and separationsbetween HIP 55507 AB and systems with young, substellar companions, our resultsopen the door to systematically measuring $\rm ^{13}CO$ and $\rm H_2^{18}O$abundances in the atmospheres of substellar or even planetary-mass companionswith similar spectral types.

Validation of elemental and isotopic abundances in late-M spectral types with the benchmark HIP 55507 AB system