Nitrogen-to-carbon ratio vs. nitrogen-to-oxygen ratio, normalised to initial values. The mixing ratios derived for HD 93840 are depicted as open dots with grey 1σ and 2σ uncertainty regions (ADS) and with error bars depicting the 1σ uncertainty (FASTWIND). Objects from our previous work are indicated: B-type main-sequence stars (Nieva & Simón-Díaz 2011; Nieva & Przybilla 2012, black dots), late O-type main-sequence stars (Aschenbrenner et al. 2023, black squares), BA-type supergiants (Przybilla et al. 2010, black diamonds), B supergiants (Paper I and II, open squares), and the stripped CN-cycled core γ Columbae (Irrgang et al. 2022, black triangle). For comparison, the development of the surface CNO abundances is shown for a 25 M_⊙, Ω_rot = 0.568 Ω_crit model (Ekström et al. 2012, red) and for a 15 M_⊙, Ω_rot = 0.95 Ω_crit model (Georgy et al. 2013, green line). The dashed and dash-dotted lines depict the analytical boundaries for the ON- and CN-cycle, respectively. A discussion is provided below.