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GCIRS 7, the brightest star in the Galactic central parsec, formed $6\pm2$Myr ago together with dozens of massive stars in a disk orbiting the centralblack-hole. It has been argued that GCIRS 7 is a pulsating body, on the basisof photometric variability. We present the first medium-resolution ($R=500$),K-band spectro-interferometric observations of GCIRS 7, using the GRAVITYinstrument with the four auxiliary telescopes of the ESO VLTI. We looked forvariations using two epochs, namely 2017 and 2019. We find GCIRS 7 to bemoderately resolved with a uniform-disk photospheric diameter of$\theta^*_\text{UD}=1.55 \pm 0.03$ mas ($R^*_\text{UD}=1368 \pm 26$ $R_\odot$)in the K-band continuum. The narrow-band uniform-disk diameter increases above2.3 $\mu$m, with a clear correlation with the CO band heads in the spectrum.This correlation is aptly modeled by a hot ($T_\text{L}=2368\pm37$ K),geometrically thin molecular shell with a diameter of$\theta_\text{L}=1.74\pm0.03$ mas, as measured in 2017. The shell diameterincreased ($\theta_\text{L}=1.89\pm0.03$ mas), while its temperature decreased($T_\text{L}=2140\pm42$ K) in 2019. In contrast, the photospheric diameter$\theta^*_\text{UD}$ and the extinction up to the photosphere of GCIRS 7($A_{\mathrm{K}_\mathrm{S}}=3.18 \pm 0.16$) have the same value withinuncertainties at the two epochs. In the context of previous interferometric andphoto-spectrometric measurements, the GRAVITY data allow for an interpretationin terms of photospheric pulsations. The photospheric diameter measured in 2017and 2019 is significantly larger than previously reported using the PIONIERinstrument ($\theta_*=1.076 \pm 0.093$ mas in 2013 in the H band). Theparameters of the photosphere and molecular shell of GCIRS 7 are comparable tothose of other red supergiants that have previously been studied usinginterferometry.

MOLsphere and pulsations of the Galactic Center's red supergiant GCIRS 7 from VLTI/GRAVITY