1.Motivation

The study of δ Scuti stars has been carried out for many years since the pioneering works of ^{Breger (1966)} and ^{Millis (1966}). It was shortly after this that the monitoring of some of these stars was started in Mexico (^{Warman, Malacara and Breger, 1974}, ^{Warman, Peña and Arellano Ferro, 1979}) and it has continued ever since.

More recently, with the acquisition of CCD detectors thirteen years ago, the observations of 𝛿 Scuti stars were retaken at the Observatorio Astronomico Nacional de Tonantzintla (TNT) with the participation of many students of the Observational Astronomy courses taught at the UNAM and later of the Escuela Latinoamericana de Astronomía Observacional (ESAOBELA). These students have provided us with motivation, and have supplied an eager workforce.

HADS stars provided excellent targets since they have short periods of pulsation, large amplitudes and, most of them have been observed for large time spans providing a unique chance to study their secular variations. Surprisingly, most of the observed stars we studied show evidence of a light travel-time effect, suggesting the presence of another invisible companion star.

What we have found is that some of these stars have lacked continuous observations and that there are large gaps in which they were not observed. Conscious of the need for long continuous observations, we developed a systematic monitoring of HADS stars at Tonantzintla to pursue our study of HADS double systems.

Aware of this need we have previously presented two lists of times of maxima of pulsating stars (^{Peña et al. 2015} and ^{Peña et al. 2017}) and here we present the third list of results from our observations.

2.Data collection

This is the third compilation of OAN results of 23 variable stars obtained from 2016 to March, 2020. These observations also include some maxima from the Observatorio Astronómico Centroamericano de Suyapa, Honduras and the Complejo Astronómico de Cota Cota, Bolivia. The results are presented for 23 stars, of which 114 maxima of pulsating stars were obtained.

The CCD reduction was done with AstroImageJ (^{Collins et al. 2017}) whereas the photoelectric observations were reduced using a classical procedure (see ^{Peña et al., 2016} for details). All times of maxima are heliocentric and were determined with a fifth grade polynomial fitting to the light curve.

The errors were determined from the RMS error of the residuals evaluated for the times of maxima, and are about 0.016 days. The accuracy of each point is given by the exposure time and varies between 3 min for the 1-meter telescope and 1 min for the smaller telescopes. It may seem contradictory to use a longer integration time for the larger aperture telescope. However, this is done because the mounting of the smaller telescopes is of an altazimuth type, which does not allow long integration times. For the 1-meter telescope there were around 40,000 counts, and for the 10-inch telescope there were 11,000 counts, enough to secure the high precision desired. The photoelectric measurements and all the light curves can be requested for inspection. The procedure followed to reduce uvby-β of the San Pedro Martir Observatory is presented in Peña et al. (2021).

In Table 1 we present the characteristics of the observed stars; that is, their IDs, coordinates (epoch 20009), V magnitude, spectral types and the observational epoch and periods in days when available from the web site of the General Catalog of Variable Stars (http://www.sai.msu.su/gcvs/cgi-bin/search.htm GCVS) as they are reported in that source. No errors are provided. All information about telescopes, photometers and filters is specified in the remarks of the table. In Table 2 the following quantities are listed: Column 1 is the ID, Column 2, date of observation, Column 3, N gives the number of data points in each run, Column 4, 𝛥t is the time span in days of the run, Column 5, the number of Tmax of the run, Column 6, the time of maxima in HJD, Column 7, the telescope, Column 8, the filter used, Column 9, the detector, Column 10 the observatory, and finally, Column 11 gives the observers and reducers. Observers and reducers are specified in the remarks at the end of the table.

Remarks (Continued). Observers.: AA: A. Artola (UNAH); ALZ: A. L. Zuñiga; ARL: A. Rentería; DSP: D. S. Piña; FS: F. Saldaña; JCC: J. Calderón; JGT: J. Guillen; JHP: J. H. Peñaa; SBJ: S. B. Ju_arez.

E17 (ESAOBELA 2017): Ramirez, Vanesa; Rodríguez, Mariana; Vargas, Stephany; Castellón, Cindy; Salgado, Ricardo; Mata, Joaquin; Santa Cruz, Raúl; Chipana, Karol; Gonzales, Lisseth; Rodríguez, Reina; De la Fuente, Diana.

E18 (ESAOBELA 2018): Calle, Carla; Huanca, Ever; Uchima, Juan Pablo; Ramírez, Raquel; Funes, Ricardo; Martinez, Juan José; Sarmiento, Karina; Cruz, Mauricio; Meza, Enith; Alvarado, Mayubell; Huaman, Victor; Ochoa, Gerson; Matamoros, Andrea.

E19 (ESAOBELA 2019): Blanco, Agustina; Benadalid, Tania; Donaire, Johany; Salazar, Luis; Quirós-Rojas, Marianela; Portllo, Alejandro; Escobar, Pablo; Mejía, Raquel; Mireles, Monica; León, Antonis; Zelada, Cindy; Báez, Sol-haret; Ng,Jessica.

E20 (ESAOBELA 2020): Carrasco, Laura; Vargas, Camilo; Barba, Miguel; Martinez, Glenda; Castellanos, Mitsa; Mejia, Nicole; Buenfil, Guadalupe; Vásquez, Franklin; Martínez, Bexy; Beato, Manuel; Paredes, Jhonnayker; Muñoz, Fernanda; Salazar, Azalea.

AOA16: Juarez, Karen; Lozano, Karen; Padilla, Artemio; Velázquez, Roberto; Santillan, Priscila.

AOA18: Bustos, Sergio; Carrillo, José Luis; Chávez, Brian; Navez, David; Zuñiga, Ana.

AOA19: Briones, Joshua; Castro, Celeste; Martínez, Fernanda; Posadas, Hilde; Romero, Mitzi; Soberanes, Hilkar; Velasco, Alex.

AOA20: Blas, Karla; Madrigal, Norma; Perez, Alexis; Ramirez, Jos_e; Santiago, Beatríz.

Bo19 (Bolivia 19): Benadalid, Tania; Donaire, Johany; Muñoz, Ruber.

Ho19 (OACS Honduras 2019): Artola, Adán; Meza, Maria Renee; Mejia, Luis Fernando.

GIEP: Meza, María Renee; Argueta, Christian