As part of a wider investigation of evolved massive stars in Galactic open clusters, we have spectroscopically identified three candidate classical Cepheids in the little-studied clusters Berkeley 51, Berkeley 55 and NGC 6603.

Using new multi-epoch photometry, we confirm that Be 51 #162 and Be 55 #107 are bona fide Cepheids, with pulsation periods of 9.83+/-0.01 d and 5.850+/-0.005 d respectively, while NGC 6603 star W2249 does not show significant photometric variability. Using the period-luminosity relationship for Cepheid variables, we determine a distance to Be 51 of 5.3(+1.0,-0.8) kpc and an age of 44(+9,-8) Myr, placing it in a sparsely-attested region of the Perseus arm. For Be 55, we find a distance of 2.2+/-0.3 kpc and age of 63(+12,-11) Myr, locating the cluster in the Local arm. Taken together with our recent discovery of a long-period Cepheid in the starburst cluster VdBH222, these represent an important increase in the number of young, massive Cepheids known in Galactic open clusters.

We also consider new Gaia (data release 2) parallaxes and proper motions for members of Be 51 and Be 55; the uncertainties on the parallaxes do not allow us to refine our distance estimates to these clusters, but the well-constrained proper motion measurements furnish further confirmation of cluster membership. However, future final Gaia parallaxes for such objects should provide valuable independent distance measurements, improving the calibration of the period-luminosity relationship, with implications for the distance ladder out to cosmological scales.

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Context Galactic starburst clusters play a twin role in astrophysics, serving as laboratories for the study of stellar physics and also delineating the structure and recent star formation history of the Milky Way.

Aims In order to exploit these opportunities we have undertaken a spectroscopic survey of the red supergiant dominated young massive clusters thought to be present at both near and far ends of the Galactic Bar.

Methods Specifically, multi-epoch observations were employed to identify and investigate stellar variability and its potential role in initiating mass loss amongst the cool super-/hypergiant populations of these aggregates.

Results Significant spectroscopic variability suggestive of radial pulsations was found for the yellow supergiant VdBH222 #505. Follow-up photometric investigations revealed modulation with a period of ~ 23.325 d; both timescale and pulsational profile are consistent with a Cepheid classification.

Conclusions #505 is one of the longest period Galactic cluster Cepheids identified to date and hence of considerable use in constraining the bright end of the period/luminosity relation at solar metallicities. In conjunction with extant photometry we infer a distance of ~ 6kpc for VdBH222 and an age of ~ 20Myr. This results in a moderate reduction in both the integrated cluster mass (2 × 10^4 solar masses) and the initial masses of the evolved cluster members (~ 10 solar masses). As such VdBH222 becomes an excellent test-bed for studying the properties of some of the lowest mass stars observed to undergo type-II supernovae. Moreover, the distance is in tension with a location of VdBH222 at the far end of the Galactic Bar. Instead a birthsite in the near 3kpc arm is suggested; providing compelling evidence of extensive recent star formation in a region of the inner Milky Way which has hitherto been thought to be devoid of such activity.

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