Context : The stellar population in the central parsec of the Galaxy is dominated in mass and number by an old ( several Gyr ) population , but young ( 6 \pm 2 Myr ) , massive stars dominate the luminosity function .
The most luminous of these stars is a M1 supergiant , GCIRS 7 .

Aims : We have studied GCIRS 7 in order to constrain the age of the recent star formation event in the Galactic Centre and to characterise it as a visibility and phase reference for observations of the Galactic Centre with the interferometric instrument GRAVITY , which will equip the Very Large Telescope Interferometer ( VLTI ) in the near future .

Methods : We present the first H-band interferometric observations of GCIRS 7 , obtained using the PIONIER visitor instrument on the VLTI using the four 8.2-m unit telescopes .
In addition , we present unpublished K-band VLTI/AMBER data and build JHKL light-curves based on archival data spanning almost 40 years , and measured the star ’ s effective temperature using SINFONI integral field spectroscopy .

Results : GCIRS 7 is marginally resolved at H-band , with a uniform-disk diameter \theta _ { \text } { UD } ( 2013 ) = 1.076 \pm 0.093 mas ( R _ { \text } { UD } ( 2013 ) = 960 \pm 92 R _ { \sun } at 8.33 \pm 0.35 kpc ) .
We detect a significant circumstellar contribution at K-band .
The star and its environment are variable in brightness and in size .
The photospheric H-band variations are well modelled with two periods : P _ { 0 } \simeq 470 \pm 10 days ( amplitude \simeq 0.64 mag ) and long secondary period P _ { \text } { LSP } \simeq 2700 — 2850 days ( amplitude \simeq 1.1 mag ) .
As measured from \@element [ ] [ 12 ] [ ] [ ] { \mathrm { CO } } equivalent width , \left < T _ { \text } { eff } \right > = 3 600 \pm 195 K .

Conclusions : The size , periods , luminosity ( \left < M _ { \text } { bol } \right > = -8.44 \pm 0.22 ) and effective temperature are consistent with an M1 supergiant with an initial mass of 22.5 \pm 2.5 M _ { \sun } and an age of 6.5 – 10 Myr ( depending on rotation ) .
This age is in remarkable agreement with most estimates for the recent star formation event in the central parsec .
Caution should be taken when using this star as a phase reference or visibility calibrator as it is variable in size , is surrounded by a variable circumstellar environment and large convection cells may form on its photosphere .