OH color-color plot for the 1612 MHz line, adapted from McBride et al. (2013). Blue squares come from the literature, collated by Baan et al. (1992a) in their Table 2(a); the black circle and the gray arrows are the detection and upper limits from McBride et al. (2013), respectively. The red arrow is the R_H ratio and R₁₆₁₂ upper limit from this work; the cyan cross shows our uncertainties; the black arrow is the upper limit calculated using the McBride et al. (2013) methodology (see Sect. 4.2). The black line shows the predicted ratios assuming equal excitation temperatures for all OH lines, with varying 1667 MHz optical depth (Lockett & Elitzur 2008; McBride et al. 2013).