1. Introduction
2. Observations
2.1. The sample
2.2. The observations
2.3. Results
3. Radiative transfer analysis
3.1. Radiative transfer model
3.2. Comparison with other studies
3.3. Description of the grid
3.4. Grid results
3.5. Discussion on the derived mass-loss rate formulae
4. Stellar parameters
4.1. Temperature, luminosity and distance
4.2. Inner envelope radius
5. Results and discussion
5.1. Mass-loss rate
5.2. $^{12}\mathrm{C}/^{13}\mathrm{C}$ isotope abundance ratio
5.3. Wind driving efficiency
6. Conclusions