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tokens: ). Moreover, due to the compactness, compact radio sources trace sightlines towards central denser regions of radial density profiles of the H i disc (Pihlström et al. 2003 ). This could be the other reason for the low detection rate in extended radio sources. Available very-long-baseline interferometry (VLBI) images for some of the compact LERG radio sources with H i absorption detection suggest that these have sizes on the scale of pc. However, we lack complete information on all sources. In HERGs, in addition to clouds in the ISM, a dusty torus in an AGN could give rise to H i absorption if the compact radio source is of a scale similar to or smaller than the torus. Figure 5. Top: log sSFR versus W2−W3. Bottom: log total stellar mass versus log sSFR, for absorption detections (filled symbols) and non-detections (empty symbols). Since H i absorption detections as well as star-formation rates are higher for WISE ‘late-type’ galaxies, it is relevant to enquire about the possible origins of the gas or fuel. It could be the result of a normal process of galaxy evolution, in which star formation started in a dusty gas-rich environment and decreased at later stages due to scarcity of fuel, combined with possible feedback from a radio source. In addition, star formation or AGN activity may get a boost from dust-rich mergers or interactions. In the literature, suggested fuelling mechanisms vary from major mergers or secular processes for HERGs to accretion of halo gas, stellar remnants, winds from evolved stars or minor mergers for LERGs (Ellison et al. 2015 ). Also, LERGs are reported with suppressed star formation in the literature (Gürkan et al. 2015 ; Ellison et al. 2016 ; Pace Salim 2016 ). Although this may be true for a majority of the LERGs in our sample, it does not explain the higher H i gas detection rate and higher rate of star formation in WISE ‘late-type’ LERGs. There have been suggestions for higher rates of mergers for WISE IR-selected AGNs with W1−W2 > 0.8 (Satyapal et al. 2014 ; Ellison et al. 2016 ). However, as can be seen from Fig. 2 , our sample hardly has any AGNs with W1−W2 > 0.8. Also the optical host galaxies do not suggest any significant morphological differences, except for a few cases of disturbed morphology or evidence of interaction with a companion galaxy, e.g. J105327.2+205836. To understand these aspects and clarify any possible evolutionary scenario, we fit a first-order linear function in log sSFR versus WISE colour for LERG detections, non-detections and also for compact and extended LERGs separately. Using the Kendall tau test, we find that there exists a significant correlation (τ = 0.47, p = 4.37 × 10 −10 ) between log sSFR and WISE W2−W3 colour (Fig. 6 ). The left panel of Fig. 6 also shows that compact radio sources (slope: 0.81 ± 0.12) and detections (slope: 0.78 ± 0.16) have similar flatter slopes, while extended sources (slope: 0.98 ± 0.19) and non-detections (slope: 0.96 ± 0.14) have similar and